In most activities, muscles of the body are required to provide forces for two functions. One primary function is to overcome resistive loads which in general include friction and gravity forces. The second primary function is to provide a force to accelerate and decelerate the body part itself plus any extra mass imposed on the body part. The proportion of these two functions varies depending upon the activity and the specific limb and joint engaged in the activity. In medical terminology, an acceleration force is produced by a concentric contraction while a deceleration force is produced by an eccentric contraction.
In throwing and racket sports, the importance of the acceleration and deceleration function of the muscles cannot be overestimated. A major league pitcher who throws a fast ball at 95 miles per hour must accelerate his arm to that speed and more before releasing the ball, and the arm must then be decelerated to avoid injury. Thus, the deceleration forces generated within a muscle can be very high.
In most known devices for exercising body muscles, the body part involved moves in one direction against a resistance, and then comes to a stop, followed by return motion to the starting point. Such exercises involve both resistive loads and acceleration and deceleration forces. In weight lifting, the resistive loads are very high. The acceleration and deceleration forces can also be high because the forces are proportional to the amount of mass being accelerated as well as to the rate of acceleration. During a bench press it has been shown that the acceleration forces can be as high as 35-40 percent of the weight being pressed. U.S. Pat. No. 3,851,874 discloses an exercise apparatus involving both resistive and acceleration/deceleration forces, wherein a cord passing through a pulley is pulled to lift a suspended weight at the other end of the cord or to stretch a spring. U.S. Pat. No. 2,396,106 discloses an exerising device including a handle joined to a weight by means of an elongate spring steel blade, so that the weight oscillates and causes vibratory stimulation of the body parts.
U.S. Pat. Nos. 3,917,281; 4,034,991 and 3,794,329, each disclose swing training apparatus in which a golf club or the like is guided by being slidably attached to a track in order to teach an accurate swing trajectory.